Wikipedia:Stack Exchange is eating our lunch

Wikipedia is currently experiencing an editor retention problem. At the same time, Stack Exchange is growing faster than ever. It may help to compare and contrast our methodology with theirs.

Reputation
On Stack Exchange, users are officially ranked by "reputation," a score which loosely measures the value each user creates for the site. Beyond a certain level, users are effectively regarded as minor deities and relative differences in rep become less important. Of course, all users are still accountable for their actions. Reputation does correlate rather strongly with participation, but it is corrected for various undesirable behaviors like spam and griefing.

On Wikipedia, users are officially not ranked at all. Unofficially, this is a joke. Users commonly use edit count and then insist that they don't. Some of them even believe it. Other measures include GA/FA involvement, barnstars, and (on the negative side) the block log. These are used to construct an underground social hierarchy. Anyone complaining about unfairness stemming from the social hierarchy is ignored because the social hierarchy does not exist. The only exception is when the evidence is too overwhelming to deny, which is rare since the hierarchy is tacit and implicit.

Automation
Stack Exchange automates most of its bureaucracy. Instead of barnstars, they have badges, which are awarded automatically upon satisfying fixed criteria. Privileges are awarded by rep. A high-rep user is nearly the same thing as a moderator. The contents of the front page are determined based on voting. Automation is also used to rate-limit or temporarily soft-block users who are risking a more permanent ban for misbehavior.

Wikipedia does use automation in some places, but normally only to work around shortcomings in the wiki software. For example, bots are used to automatically archive discussions or implement large scale article changes, after humans have decided what should be done in every particular. The only area where bots are allowed a significant degree of automatic decision making is anti-vandalism efforts, and then only because there's no other reasonable way to fight vandalism. Any effort to automate (for example) RFA or even AIV (one of the most mechanical noticeboards on the site, significantly moreso than AN3) would die in committee.

Ownership of content
On Stack Exchange, every question and every answer belongs to a particular user. This user's name and avatar appear prominently. It is possible to edit someone else's content, after a rep threshold or with peer review, but users are strongly discouraged from changing the intent or substantial meaning in this case. Some posts are ownerless, labeled "community wiki," but this is the exception rather than the norm. It is typically used when posting material which should not earn rep (such as someone else's idea, properly attributed). If you disagree with someone else, the correct course of action is to post your own content and let the votes decide.

On Wikipedia, everything is formally ownerless and informally politicized as hell. While in principle it is permissible and even encouraged to make large scale changes to, for instance, Eastern Europe, this is actually a very bad idea unless you enjoy walking on eggshells.

Voting
Stack Exchange uses voting to make most decisions. Votes determine rep, prominence of content, agreement in meta discussions, and which high-rep users get promoted to a moderator seat following an election. Voting is also used to decide which posts should be "closed" or deleted outright, and to approve edits by low-rep users.

Wikipedia uses consensus. Generally, this either produces the same result that discussion followed by voting would have produced, or it creates an inconclusive mess that nobody knows how to deal with. An administrator (or bureaucrat, for RFA/B) then either closes in favor of the status quo or picks the side which better conforms to Wikipedia's groupthink. When this process inevitably falls apart, ArbCom solves the problem by banning everyone. OK, not literally everyone, but most ArbCom cases these days do end with "So-and-so is (topic-)banned for a period of N months/1 year" repeated for most of the major parties.

Rules
Most of Stack Exchange's rules are fairly obvious and can be deduced from lurking or from general netiquette (e.g. don't randomly flame people). All of the rules can be read from one handy-dandy web page. Note that this page also covers quite a lot of "how the site works" information and basic support questions like "what if I lose my password?"; there are relatively few actual rules.

Wikipedia has a much longer list of policies and guidelines, many of which are quite unobvious but nevertheless enforced, such as 3RR or GNG. The non-free content rules are obnoxiously bureaucratic, while smaller wikis often just slap a generic fair-use template on each image and call it a day.